This invention relates to the field of high density boards and the method of manufacture of high density boards. More particularly, this invention relates to high density insulation boards and the method of manufacture thereof, particularly boards intended for use as insulating spacers in electrical power transformers.
As is well known, electrical power transformers having core form transformer coil assemblies require board material to serve as radial spacers and insulators. This board material, which is usually cellulose fiberboard, must meet certain requirements for compressibility properties. In particular, low deformation under load, i.e. low percentage compression, is the compressive property of particular interest. If the transformer board does not possess this desirable characteristic, the coil assemblies of the transformer are not dimensionally stable and performance characteristics of the transformer may be adversely affected by changes in the dimensional relationship between the coil assemblies as they move closer together upon compression of the spacer boards. In particular, a significant physical load is imposed on the boards by the coils when the transformer is operated. If the board does not resist deformation under the loading from the coils, the desired tightness and rigidity of the coils is lost; the coils become loose, and the coils may fail due to physical vibration or may come into contact and short out.
It has been believed in the art that desirable compressive properties for this insulating board are related to the density of the board, with the compressive properties improving, i.e. being more desirable, with increasing density of the board. This relationship between compressive properties and densities immediately suggests that desired compressive properties can be achieved merely by increasing the density of the board. While that may be generally true, there are, however, both practical and economic reasons which impose upper limits on the density of the board.
Several techniques have been known in the prior art for manufacturing the transformer board. In most or all of the techniques, multi-ply board is initially formed in conventional paper making manner on a wet cylinder machine; and the board in its initial wet form contains about 60% water, this being referred to as wet board. In one known prior art technique, the wet board is pressed in a cold press to remove, i.e. squeeze out, about half the water; the board is then dried in a tunnel dryer with circulating hot air to reduce the moisture content to about 6%; and the board is then calendered to density. In another prior art technique, the process described immediately above is followed with the exception that the cold pressing step is omitted and all drying occurs in the tunnel dryer. In a third known prior art technique, the wet multi-ply board is placed in a press between upper and lower sheets of woven wire fabric. The board is then pressed between the platens of the press which are steam heated, and the board is retained under pressure until the moisture content is reduced to a desired level. Moisture is removed both by the physical pressing of the board and by vaporization caused by the steam heated platens, the fluid escaping through the woven wire fabric on either side of the board. The compressive characteristics of board produced by this third technique of the prior art are somewhat superior to the compressive characteristics of the board of the same density produced by the first and second described techniques of the prior art.
As indicated above, it was previously believed that the compressive properties of the boards produced by these prior art techniques could be improved only by increasing the density of these boards. However, as previously discussed, there are both practical and economic limitations on the density values that can be achieved for the board.